This invention relates to a device for purifying an exhaust gas and, more particularly, to such a device which can be used effectively against an exhaust gas from an automobile engine.
In order for an exhaust gas purifier using a catalyst to be able to function effectively, the temperature of the catalyst must be above its activation temperature. For this reason, there have been various attempts to provide a means for heating the catalyst such that it can start functioning quickly as an effective catalyst even during the start-up period of an engine when the temperature of the exhaust gas therefrom is still relatively low.
For example, it has been proposed to use a metal as a catalyst substrate and to pass an electric current therethrough to heat and activate the catalyst supported on its surface. FIG. 6A shows a device 61 based on this idea, characterized as comprising a metallic substrate 62, electrodes 68 and an electric power source 69. The substrate 62 has many longitudinally oriented through holes and hence is commonly referred to as a honeycomb. The arrow shows the direction in which an exhaust gas to be purified is passed through the device 61. As shown in FIG. 6B, a catalyst layer 64 is formed directly on the inner surfaces of the honeycomb 62. As power is supplied from the power source 69 to the honeycomb 62 through the electrodes 68, the electrically conductive metallic honeycomb 62 serves as a portion of a closed circuit and becomes heated due to its own resistance. The temperature of the catalyst layer 64 on its inner surface is raised accordingly, reaching the activation temperature of the catalyst, such that harmful components of the exhaust gas such as CO, NO.sub.x and HC are removed by the catalyst layer 64 as the exhaust gas passes through the throughholes of the honeycomb 62.
A prior art device thus structured has several problems. Firstly, since the metallic honeycomb has low resistance and cannot function effectively as a heater, the catalyst carried thereon cannot be heated efficiently and its temperature cannot be raised quickly. Secondly, temperature does not become uniform throughout the catalyst layer when the exhaust gas is passed through. As a result, the effect of purification becomes variable from one position to another where the exhaust gas passes. In the case of a honeycomb with a circular cross-sectional shape, for example, the flow of the gas is fast near the center of the honeycomb and slow near its peripheries. Consequently, the temperature becomes lower near the center due to heat exchange, and the catalyst cannot perform its function effectively.